The sudden departure has unsettled residents of this area along the Syrian border with Turkey. Many followed the U.S. troops out of town. Others remained, left to decide whether they, too, should vacate before Turkey invades.

“They could strike at any moment,” said Ibtisam al-Subhi, 30, who lives next to the outpost. She said she had decided to stay despite the danger. “We must stand up for ourselves,” she said. “These are our lands; they are not Turkish.”…